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is not already in use and then set it to your device.  Each router configuration is different, but either make it a static IP address or reserve it or what ever naming convention your router uses to assign a fixed IP to a device.  

2)  change the code in the header of the example code to match the IP address you assigned to your board.  I assigned mine to as shown above.  This must be hard coded into the software as shown below.  You must also hard code the IP address for your gateway.  You may also need to change the sub-domain mask and MAC address but I didn't.  I just used the default with no issues.  Finally, you may want to change the port number.  The normal access is HTTP which is typically port 80 which is the default.  This lets you use a standard web browser to access you device.  However there may be times you want to access differently.  In that case you may wish to use a different port.  I used port 80 which was as the example code provided.
byte ip[] = { 192, 168, 0, 90 }; //ip address to assign the Arduino
byte gateway[] = { 192, 168, 0, 1 }; //ip address of the gateway or router

//Rarely need to change this
byte subnet[] = { 255, 255, 255, 0 };

// if need to change the MAC address (Very Rare)
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };

Server server = Server(80); //port 80

3)  If you are intent on using this only inside your home network, then you are set.  But if you want access remotely, you'll need to enable port forwarding on your router.  Most routers support this but it is disabled.  You will need to change your port forwarding to forward external port 80 access to internal port 80 (or the port numbers that work for you) at the internal IP address that you use.  Remotely, you will use the IP address assigned by your Internet service provider and port 80.  This will be referred internally to the static IP address assigned to your board and the correct port number.  Usually these settings are a part of the firewall section of your router.  Be aware that enabling this opens your network to external traffic so be sure that any computers on the network have current Internet protection software.  

4)  Unfortunately, most home users do not receive a fixed external IP address from their ISP.  It is changed periodically.  Therefore services such as DYNDNS and TMO may be used to periodically check your actual IP